QAM is well known in communications art and combines characteristics of both phase modulation and amplitude modulation to reduce the bandwidth required to carry a certain amount of information in an information-bearing signal. In QAM, information is conveyed using changes in both the amplitude of a carrier wave and the relative phase angle of the carrier signal with respect to a reference angle. Using QAM modulation to convey digital data, 2, 3, 4, or more, bits of digital information can be conveyed per QAM signal element.
Multi-carrier QAM is a technique in which an information-bearing signal, such as serial digitized voice, digital data from a computer or other machine for example, is divided up into multiple, separate, frequency division multiplexed QAM signals. Each QAM signal occupies a discrete frequency band (with each of the bands being substantially frequency adjacent to the others) and carries a portion of the information in the information-bearing signal.
A problem with QAM-based communications systems, including multi-carrier QAM, is consistently and coherently demodulating information in a QAM signal. A QAM signal conveys information using both the amplitude of a carrier wave and the phase angle. The magnitude and phase angles of a carrier, are represented as vectors (which have a magnitude and phase angle with respect to some reference axis) that point to various points or loci on a Cartesian plane, each locus on the plane identifies a particular binary value. Each vector can be represented as a carrier having a particular amplitude level with a carrier signal at some distinct phase angle.
For example, a vector having a unit length of one at a forty five degree angle with respect to the x-axis might "point" to a point identified as representing a binary value or pattern of 0010. A vector with a unit length of one-half and at forty five degrees might point to a point identified as representing a binary value of 0110. A vector with length equal to one, at zero degrees might represent 0000, and so on. The relative magnitude and phase angle of a carrier signal correspond to the relative magnitude and phase of a vector that points to a particular point in a plane, which represents binary values assigned to the point. (A transmitted signal, that represents a vector that points to a particular point on a plane, which point is established to represent some binary value, is detected, demodulated and decoded by the receiver to yield the binary value represented by the vector.) Vectors of varying magnitude and phase angles can represent multiple binary values. (The number of discernible amplitudes and phase angles will increase the number of bits of information representable by each QAM signal element. Increasing the number of possible amplitude levels and decreasing phase angle differences will increase the transmitter power required.) Sending streams of vectors, represented as bursts of amplitude and phase-modulated RF carrier, is a way of sending streams of digital information. (Multilevel QAM is well known art. See for example "All About Modems" copyright 1981 by Universal Data Systems, Inc. or other digital communications texts.)
To coherently detect information from QAM elements in a QAM signal, a receiver must be able to accurately differentiate between amplitude variations in the carrier wave as well as phase angle changes. In many environments, phase jitter or phase shift may accompany fading and multi-path signal propagation. A receiver must be able to reliably detect phase angle changes and carrier amplitude despite fading and multi-path propagation. When multi-carrier QAM is used, each QAM signal may experience its own fading requiring that each QAM signal have its own synchronizing sequence.
A synchronizing sequence of QAM signal elements that permit a receiver to synchronize to, or lock up with, the QAM transmitter may assist the receiver in locating the relative timing of an information stream. Arbitrary synchronizing sequences for a QAM receiver may provide no real benefit however, if the synchronizing sequence requires relatively complex computational activities to be carried on by the receiver. A QAM communications system that simplifies the complexity of a QAM receiver would be an improvement over the prior art.
In a multi-carrier QAM information system it may be necessary to provide synchronization information in each of the subchannels to permit a receiver to coherently detect information in each subchannel. A synchronizing sequence that is adapted for use with multichannel QAM would be an improvement over the prior art. A QAM receiver that must detect the synchronizing sequences might be simplified if the synchronizing sequences are chosen to reduce computational complexity required to identify signalling sequences in multiple QAM channels. Other benefits from a preferred synchronizing sequence are also realized as well, including simplified automatic frequency control for the receiver IF stages and improved synchronization timing.